Variable dwell ignition system

ABSTRACT

An electronic triggering circuit for an ignition system develops electric signals which correspond to the closing and opening of breaker points to supply variable dwell (ratio of on-to-off) pulses to the primary winding of the ignition coil at speeds below a predetermined RPM. The circuit operates to supply constant dwell pulses to the primary winding of the ingition coil at higher engine speeds.

United States Patent [19] Weber [11] 3,831,571 Aug. 27, 1974 VARIABLEDWELL IGNITION SYSTEM [75] Inventor: Howard F. Weber, Scottsdale, Ariz.[73] Assignee: Motorola, Inc., Franklin Park, Ill. [22] Filed: May 11,1973 [21] App]. No.: 359,472

3,749,974 7/1973 Kissel 315/209 T Primary Examiner-Laurence M. GoodridgeAssistant ExaminerRonald B. Cox Attorney, Agent, or Firm-Mueller,Aichele & Ptak ABSTRACT An electronic triggering circuit for an ignitionsystem [52] US. Cl 123/148 E, 315/209 R develops electric s gnal whichrr spond t th los- [51] Int. Cl. F020 1/00 g and p n ng of r ak r pointsto supply variable [58] Field ofSearch 123/148 E; 315/209 T, dwell(ratio of on-to-off) pulses to the primary wind- 315 /2Q9 SC ing of theignition coil at speeds below a predetermined RPM. The circuit operatesto supply constant [56] References Cited dwell pulses to the primarywinding of the ingition coil UNITED STATES PATENTS at ig er enginespeeds. 3,587,552 6/1971 Varaut 123/148 6 Claims, 3 Drawing Figures3,605,713 9/1971 LeMasters 123/148 E 42 T H 2:25? Y j 24 26 IO HIGH iAND TIME AND V LTAG TRIGGER I4 4; 7

ug E CURRENT REF LIMI TE R PATENTED M 2 3.3%]... 57 1 MI 1 Bf 2 V2 FIG/70% DUTY II CYCLE COIL 22 24 26 I0 I v HIGH I AND TIME AND I MoNo DRIvEVOLTAGE 7 a GATE MULTIPl-Y GATE SMTCH TRIGGER I4 4 0 V TIME cuRRENTLIMITER REE I F/G3 CYCLE OF CIRCUIT oPERATIoN /\ICONSTANT TIME MoNoPULSE B C A DISCHARGE c 36 CHARGE OUTPUT OF TRANsIsToR E 50 l TIMETRANSISTOR 74' I I Is OFF I F I I c 78 CHARGE G l H I I j /c 7sDISCHARGE L OUTPUT OF I TRANsIsToR so i I con. TuRN ON TIME L J l h TIMEPAIENTEDmazmu saw an z M ETFMOZOE 1 VARIABLE DWELL IGNITION SYSTEMBACKGROUND OF THE INVENTION The Kettering ignition system currently usedin vehicles depends upon the energy storage in the primary of a highturns ratio ignition coil to develop the necessary output voltage tofire the spark plug. This energy level is dependent upon the coilcurrent flowing at the time the coil circuit is interrupted by thebreaker points to deliver output spark voltage. The coil current thatcan be reached during the available time is dependent upon coil primaryinductance, primary resistance and voltage.

Variable dwell transistorized ignition circuits operating on theKettering principle have been proposed in which the current through theprimary winding of the ignition coil is turned on only shortly beforethe ignition point and is turned off at the moment the ignition pulse isdesired. At low engine speeds, that is when the pickup pulse source fortriggering the ignition system operates at a relatively low frequency,the current is connected to the ignition coil long before the ignitiontime and a strong ignition pulse is provided. At higher speeds, however,the frequency of the triggering pulses is increased and the current isconnected to the ignition coil for increasingly shorter periods of timeso that the ignition impulse to the coil is weaker at high speeds sothat ignition degrades with increased engine speed.

Some prior art electronic ignition systems have provided a constant offtime for the coil current, causing an undesirable power dissipation atlow speeds or low RPM of operation of the engine. Thisis highlyundesirable and creates a heavy drain on the battery of the vehicle inwhich the ignition system is used. I

It is desirable to employ an ignition system which does not waste powerat low RPM and which employs a variable dwell, or ratio of on-time tooff-time, which varies in a manner to cause the on time (during whichcharging current flows through the primary winding of the coil) to berelatively constant up to some preestablished speed of the engine; andwhich employs a constant dwell at engine speeds above thispreestablished speed in order to improve the performance of transistorignition systems over those previously known.

SUMMARY OF THE INVENTION Accordingly, it is an object of this inventionto pro- In accordance with a preferred embodiment of this invention,input pulses obtained from a magnetic pickup are applied to a monostablepulse generator which produces a train of pulses, each having a fixedduration and occurring at a frequency determined by the RPMs of theinternal combustion engine. The output pulses from the monostable pulsegenerator are applied to a 70 percent duty cycle circuit to initiate acycle of operation of that circuit. The output of the percent duty cyclecircuit and the output of the monostable pulse generator are applied toa time multiplying circuit which produces a variable inhibiting output,the duration of which is a function of the relationship of the width ofthe pulses from the monostable pulse generator and the output of the 70percent duty cycle circuit. The outputs of the 70 percent duty cyclecircuit and the time multiplying circuit are applied to a coincidencegate which, after termination of the inhibiting signal from the timemultiplying circuit, passes the 70 percent duty cycle circuit output toa drive circuit to permit the conduction of direct current through theignition coil. Thus, the dwell of the signals applied to the drivecircuit and, therefore, to the ignition. coil is variable in accordancewith the joint operation of the time multiplying circuit and the 70percent duty cycle circuit. As the engine speed increases, a speed isreached where the pulse width of the pulses from the monostable pulsegenerator exceed the 30% time of the output of the duty cycle circuit.Then for that and higher speeds, there is no inhibiting signal obtainedfrom the multiplying circuit and the circuit operates as a constantdwell circuit.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of apreferred embodiment of the invention;

FIG. 2 is a detailed schematic drawing of the circuit shown in FIG. 1;and

FIG. 3 illustrates various waveforms useful in explaining the operationof the circuits shown in FIGS. 1 and 2.

DETAILED DESCRIPTION Referring now to the drawings, there is shown inFIGS. 1 and 2 a transistor ignition system operating as a variable dwell(variable ratio of on timeto off time of current'flow in the primarywinding of the ignition coil of an automobile) ignition system whichconverts to a constant dwell ignition system at a predetermined enginespeed and continues to operate as a constant dwell ignition system atspeeds above such predetermined speed. i

FIG. 1 illustrates in block diagram form the circuit components whichare used to provide this variable dwell to constant dwell circuitoperation. Preferably, a magnetic pickup device (not shown) ispositioned within the distributor of a vehicle to produce a sequence oftrigger pulses which are applied to an input terminal 10 of the ignitionsystem shown in FIGS. 1 and 2 to control the operation of the system. Asuitable pickup device which can be used to produce the trigger pulsesto a pulse input terminal 10 may be of the type disclosed in US. Pat.No. 3,390,668, issued to Arthur G. Hufton and assigned to the sameassignee of the present application.

Each input trigger pulse triggers a monostable multivibrator 11 into itsastable state to produce an output pulse as shown in waveform B of FIG.3. This output pulse initiates a cycle of operation of a 70 percent dutycycle circuit 12, which preferably is a constant duty cycle circuit ofthe type disclosed in copending application Ser. No. 308,125, filed Nov.20, 1972, and assigned to the same assignee of the present application.

percent duty cycle circuit 12 is still in its off condition or in the 30percent portion of its cycle. If this condition exists, the timemultiplying circuit 16 is provided with an enabling output from the ANDgate 14 for a time period which extends from the termination of theinput pulse from the monostable multivibrator l1 until the 70 percentduty cycle circuit 12 changes to its on state. This time interval isvariable since the output pulses from the monostable multivibrator 11are of fixed duration whereas the total time interval between cycles ofoperation of the 70 percent duty cycle circuit 12 is longer at low speedoperation of the engine and becomes increasingly shorter at higher speedoperation.

The time multiplying circuit 16 produces an output inhibiting signalwhen the 70 percent duty cycle circuit 12 switches to its on state. Theduration of the inhibiting signal is a multiple of the time intervalwhich occurred between the end of the monostable output pulse and theturning on of the 70 percent duty cycle circuit 12.

This inhibiting signal from the time multiplying circuit 16 is appliedto an AND gate 18 along with the output signal from the 70 percent dutycycle circuit 12. The AND gate 18 also is provided with a third inputfrom a current limiter circuit 20, and this third input normally is anenabling input. The output of the AND gate 18 is supplied to a drivecircuit 22, which in turn controls the conduction of a high voltageswitch 24 coupled to the primary winding of the ignition coil at aterminal 26.

No current flows through the coil from the terminal 26 until the switch24 is switched on by the drive circuit 22. This occurs only when allthree inputs to the AND gate 18 are enabling inputs. Such a conditionexists when the 70 percent duty cycle 12 is in its on" state and theinhibiting output from the time multiplying circuit l6 terminates. Whenthis occurs, the AND gate 18 causes the drive circuit 22 to turn on thehigh voltage switch 24 and current flows through the coil. Thiscondition exists until the next pulse from the monostable multivibratorcircuit 1] occurs at which time the cycle of operation repeats.

At some high speed the of time of the 70 percent duty cycle circuit 12becomes equal to the duration of the monostable pulse width. At this andhigher speeds of the engine, the time multiplying circuit 16 is rendercdineffective and no inhibiting signals are supplied by that circuit. Theoutput of the AND gate 18 then follows the constant dwell output signalsfrom the 70 percent duty cycle circuit 12 to operate the drive circuit22 and the high voltage switch 24 as a constant dwell circuit for suchhigh engine speeds.

After the last trigger pulse appears on the input terminal to themonostable multivibrator 11, the circuit operates to predict theoccurrence of another trigger pulse and the high voltage switch 24remains conductive. This causes current to be continuously suppliedthrough the coil from the terminal 26. If no trigger pulse appears onthe terminal 10 to tum off the coil circuit, the transistors in the highvoltage switch must dissipate high power for a long period of time. Thisis undesirable. Thus, it is necessary to turn off the high voltageswitch 24 if the time interval between successive pulses on the terminal10 exceeds the longest interval which would occur in normal operation.For this reason, the output of the monostable multivibrator 11 also isapplied to a time limiter reference circuit 28 which is continuouslyreset by the output pulses from the monostable circuit 11.

If the time interval between output pulses from monostable circuit 11exceeds the maximum amount which should occur in operation of thesystem, the time limiter reference circuit 28 causes a current limitercircuit 20 to produce a gradually increasing inhibiting signal to theAND gate 18. A gradual or slow reduction in the output of the drivecircuit 22 results, which in turn relatively slowly turns off the highvoltage switch 24. The current limiter circuit 20 also operates inresponse to the current flowing through the switch 24 to limit themaximum current by reducing the drive circuit output through the gate 18whenever such maximum current is sensed.

It should be noted that although the above description refers to theconstant duty cycle circuit 12 as a percent duty cycle circuit, thispercentage is an arbitrary one and can be varied in accordance with theparticular operating conditions desired in actual applications of thecircuit. A 70 percent duty cycle operation for the circuit 12 is onewhich typically is within the range of operation which would beencountered.

The circuit described in conjunction with the block diagram of FIG. 1can be implemented in monolithic integrated circuit form as illustratedin FIG. 2 and the operation of the detailed schematic circuit shown inFIG. 2 is given in conjunction with the waveforms shown in FIG. 3 for abetter understanding of the system.

Referring now to FIG. 2, the various portions of the detailed schematicdiagram depicted therein are provided with the reference numbers whichcorrespond to the circuit functions of FIG. 1 to facilitate correlationbetween the circuits of FIG. 2 and FIG. 1.

In FIG. 3 waveform A shows the time period for a single cycle of thecircuit operation of the circuit shown in FIGS. 1 and 2. This cycle isnot a complete cycle of the rotor of the distributor but represents thecycle .required to produce each individual spark in the firing sequencefor operating the internal combustion engine with which the circuit isused. The cycle of circuit operation is not of a fixed time duration butis longer for low speed operation of the engine and is shorter for highspeed operation. Thus, the time duration which is indicated in FIG. 3 iscorrect for only a single speed of operation of the engine. It is to beunderstood that this time frame can be greater or less than that whichis illustrated.

The pulse of waveform B is the only pulse of fixed duration which isillustrated in the waveforms of FIG. 3. All of the other time periodsillustrated vary with respect to the monostable output pulse of waveformB depending upon the speed of operation of the engine.

The pulses B from the output of the monostable multivibrator 11 areapplied through isolating resistors 30 and 32 to the inputs of the 70percent duty cycle circuit 12 and the time multiplying circuit 16. TheAND gate 14 of FIG. 1 is illustrated as a junction 14 in FIG. 2 sincethis gate is not a true logic AND gate, but instead comprises a pair ofanalog inputs to the time multiplying circuit 16 of FIG. 2. Thefunctional operation of this portion of the circuit of FIG. 2, however,is the same as the portion of the circuit of FIG. 1 which has beendescribed in conjunction with the AND gate 14.

When the positive output pulse of the monostable multivibrator circuit11 is applied to the input of the constant duty cycle circuit 12, itcauses an input transistor 34 to be rendered conductive to initiate adischarge cycle of a timing capacitor 36. This discharge cycle isillustrated in waveform C of FIG. 3. The rate of the discharge iscontrolled by a current source consisting of a PNP transistor 38connected in series with a current limiting resistor 40 to a source ofpositive potential (not shown) on the positive battery tenninal 42. Thetransistor 34 completes the discharge path to ground. The value of theresistor 40 and the bias on the base of the transistor 38 determine therate of discharge, and the parameters of these circuit components can bechanged to vary the rate of discharge.

When the transistor 34 is rendered conductive to commence the dischargeof the capacitor 36, the current flow through the capacitor 36 isreversed from the direction which it previously was flowing and causes anegative bias to be applied to the base of an output transistor 44 forthe constant duty cycle circuit 12. The transistor 44 is then renderednonconductive and the potential on its collector rises to near the fullpositive potential available on the terminal 42 which is coupled to thecollector of the transistor 44 through a collector load resistor 46.This positive potential is fed back through a coupling resistor 48 tothe base of the transistor 34 to maintain the transistor 34 conductivefollowing termination of the input pulse from the monostablemultivibrator 11. This is illustrated by a comparison of waveforms B andC of FIG. 3 which shows that the capacitor 36 continues to dischargethrough the transistor 34 after termination of the pulse shown inwaveform B.

When the transistor 44 is rendered nonconductive, an NPN transistor 50coupled to the collector of the transistor 33 is rendered conductive tocause a near ground potential. to appear on its collector. The collectorof the transistor 50 is connected through an isolating resistor 52 tothe junction 14, so that as long as a positive pulse appears at theoutput of the monostable multivibrator 11, the junction 14 remains at apositive potential.

After the capacitor 36 has completed discharging to the point where thebase of the transistor 44 is forward biased relative to its emitter, thetransistor 44 then is rendered conductive. The drop in potential on thecollector of the transistor 44 fed back to the base of the transistor 34once again renders the transistor 34 nonconductive. The capacitor 36commences charging in the opposite direction, at a rate controlled bythe parameters of a PNP current source transistor 54 and a resistor 56,through the base-emitter junction of the transistor 44. This chargingrate is illustrated in waveform D of FIG. 3; and for the purposes of thediscussion of the preferred embodiment, the charge rate is selected tobe approximately percent of the total timing cycle shown in waveform A,while the discharge rate of the capacitor 36 comprises 30 percent ofthat cycle. This ratio is determined by selection of the relative valuesof the resistors 40 and 56, with the resistor 56 having the higherresistance in the example given.

Control of the current conduction of the current source transistors 38and 34 is effected by a divider circuit consisting of a resistor 64, aresistor 58, a PNP transistor diode 60 and another resistor 62 connectedin series between the source of positive potential and ground. Inaddition to providing the bias for the current source transistors 38 and54, this circuit also supplies a corresponding bias to an additionalpair of PNP current source transistors 66 and 68 in the time multiplyingcircuit 16. The transistors 66 and 68 operate in a manner similar to theoperation of the transistors 54 and 38 in the constant duty cyclecircuit 12 and supply currents of values determined by the relativevalues of a pair of resistors 70 and 72 connected in series with thetransistors 66 and 68, respectively, to the resistor 64.

The time multiplier circuit 16 is similar in operation to the operationof the constant duty cycle circuit 12 and includes an input transistor74 and an output transistor 76 which correspond functionally to thetransistors 34 and 44, respectively. ln the circuit 16, however, thereis no feedback from the collector of the transistor 76 to the base ofthey transistor 74; so that the conductivity of the transistor 74 isdetermined solely by the relative values of potential applied to itsbase and emitter. The equilibrium state of the time multiplying circuit16 just prior to the application of each pulse from the monostablemultivibrator l 1 is a state in which both the transistors 74 and 76 areconductive and the charge storage capacitor 78 is at an equilibriumcondition (the same potential at both ends), with no charging takingplace in either direction. During the time interval when the transistor44 is conductive, the transistor 50 is non conductive. This causes arelatively high positive potential to appear on its collector, and thispotential is applied by way of the resistor 52 to the base of thetransistor 74 so that the transistor 74 is held conductive.

When the next pulse from the monostable multivibrator 11 is applied tothe base of the transistor 34, the transistor 44 becomes nonconductiveand causes the transistor 50 to be conductive to drop the potential onthe collector thereof to near ground potential. This does not cause thetransistor 74 to be made nonconductive at this time, however, since thepositive pulse from the monostable multivibrator also is applied to thejunction 14 simultaneously with its application to the base of thetransistor 34. Thus, there is no change in the state of operation of thetime multiplying circuit 16 so long as the pulse from the monostablemultivibrator circuit 11 remains. At the time the pulse from themonostable multivibrator circuit 11 terminates, however, the transistor74 is rendered nonconductive provided that the transistors 34 and 50also are nonconductive at this time. This is true so long as thecapacitor 36 is in the discharge cycle of operation illustrated inwaveform C. Thus, the transistor 74 is rendered nonconductive, asindicated in waveform F from the time that the pulse from the monostablemultivibrator (waveform B) terminates until the next charge cycle of thecapacitor 36 begins (waveform D).

During the time that the transistor 74 is rendered non-conductive, thecapacitor 78 is charged through a charge circuit including the resistor70, the current source transistor 66, and the base-emitter junction ofthe transistor 76. When the transistor 76 is conductive, the potentialon its collector is near ground potential and an output transistor 80for the time multiplier circuit is rendered nonconductive causing thepotential on its collector to be high. This is indicated in the initialportion of waveform I which illustrates the output potential on thecollector of the transistor 80.

When the capacitor 36 of the constant duty cycle circuit l2 commencescharging, the output transistor 50 for the constant duty cycle circuitagain is rendered nonconductive, causing a positive potential to appearon its collector. This in turn causes the transistor 74 once again to berendered conductive initiating the discharge cycle of the capacitor 78which is illustrated in waveform H of FIG. 3. When this discharge cyclecommences it causes the bias on the base of the transistor 76 to benegative with respect to the ground potential on its emitter, therebydriving the transistor 76 to a nonconductive state and rendering thetransistor 80 conductive. This latter condition is illustrated in thecenter portion of waveform I.

The capacitor 78 discharges at a rate determined by the parameters of adischarge circuit including the current source transistor 78 and theresistor 72. As illustrated in waveforms G and H, the rate of dischargeof the capacitor 78 is indicated as longer than the rate of charge fromthe same potential. The total length of time for the capacitor 78 todischarge to the point where the base of the transistor 76 once again isforward biased relative to its emitter is determined both by the rate ofdischarge and by the final charge which the capacitor 78 reached duringthe time interval that the transistor 74 was nonconductive. This finalcharge level varies in accordance with the duration of time that thetransistor 74 conducts, so that the total discharge time period alsovaries in accordance with the maximum charge reached by the capacitor 78during the charge portion of the cycle of operation.

For high speed operation of the engine, the constant duty cycle circuitultimately reaches a point where the length of time to discharge thecapacitor 36, as shown in waveform C, becomes equal to or less than thefixed width or time duration of the pulse from the output of themonostable multivibrator shown in waveform B. When this occurs, there isno time when the transistor 74 is rendered nonconductive since therethen is a continuous overlap between the pulses from the monstablemultivibrator 11 and the positive output of the transistor 50 applied tothe junction 14. Then the capacitor 78 always is at an equilibrium statein which both the transistors 74 and 76 are conductive, and thetransistor 80 is continuously non-conductive. In such a situation, thewaveform I then is continuously at the positive potential throughout theentire cycle of operation. This only occurs for a predetermined speed ofthe engine relative to the width of the output pulses from themonostable multivibrator. The significance of this operation will becomeapparent from the subsequent description of the operation of theremainder of the circuit.

As stated above in conjunction with the description of operation of theblock diagram circuit in FIG. 1, it can be seen that the output of theconstant duty cycle circuit 12 and the output of the time multiplyingcircuit 16 both are applied to respective inputs of an AND gate 18. ThatAND gate 18 is illustrated in' FIG. 2 as comprising three diodes 82, 84,and 86. The diode 86 is connected to the output of the current limitercircuit 20, the operation of which will be described subsequently. Atthe present time, assume that the diode 86 is back biased with apositive potential applied to its cathode. This operates to enable theAND gate 18.

Whenever all three diodes 82, 84 and 86 are reverse biased with positivepotentials applied to their cathodes, a positive potential is appliedfrom the output of the AND gate 18 to forward bias an NPN inputtransistor. Thus, the transistor 88 is rendered conductive only whenboth the transistors and 80 are nonconductive. Examination of waveformsE and I indicates that this occurs only during the time intervalindicated in waveform J as coil turn-on time.

The circuit operation can be considered to be such that the primarycontrol of the conduction of the input transistor 88 in the drivecircuit 22 is obtained from the collector of the transistor 50 in theconstant duty cycle circuit 12. In the absence of the time multiplyingcircuit 16, the transistor 88 would be rendered conductive for percentof the duty cycle of operation established by the circuit 12.

The discharge time interval for the capacitor 78, however, operates tocause the transistor to be conductive during a portion of the time thatthe output transistor 50 is non-conductive. This causes a near groundpotential to be applied through the diode 84 to the base of thetransistor 88 causing it to remain nonconductive until the capacitor 78discharges to the level where the transistor 76 becomes conductive and Ithe transistor 80 once again becomes nonconductive,

as indicated in waveforms H and I. Thus, the time multiplying circuit16, through the AND gate 18, inhibits the turning on of current throughthe primary winding of an ignition coil 90 connected to the terminal 26until the discharge of the capacitor 78 is complete.

The particular form of the drive circuit 22 and high voltage switch 24which control the conduction through the ignition coil 90 is notimportant, and the circuit which is shown in FIG. 2 is illustrative ofthe type of circuit which can be used. So long as the emitter-followertransistor 88 is nonconductive, an NPN transistor 92 controlled by thetransistor 88 also is nonconductive. The collector of the transistor 92is coupled to the base of a transistor 94 which then is renderedconductive for this state of operation. This in turn causes an NPNemitter follower transistor 95 to be rendered nonconductive. The emitterfollower transistor 95 is coupled to the high voltage NPN switchingtransistor 24 to render that transistor nonconductive so long as thetransistor 88 is nonconductive.

When the transistor 88 conducts, the conductive states of all of thetransistors 92, 94, 95 and 24 then change. The transistor 92 conducts,and the transistor 94 is rendered nonconductive which in turn causesboth of the transistors 95 and 24 to conduct. When the high voltageswitching transistor 24 conducts, current flows from the terminal 42through the primary winding of the ignition coil 90, the terminal 26 andthe transistor 24 through its emitter resistor 96 to ground. The

duration of time over which this current flows is illustrated inwaveform J.

The circuit continues in this state of operation until the next pulsefrom the monostable multivibrator 11 occurs. Then the output state ofthe constant duty cycle circuit 12 changes to cause ground potential tobe applied through the diode 82 from the transistor 50 to the base ofthe transistor 88 causing it to become nonconductive and the highvoltage switch transistor 24 once again is rendered nonconductive. Thecollapse of flux which then occurs in the primary winding of theignition coil 90 is applied to the secondary winding to produce thedesired spark.

The time duration during which current flows through the primary windingof the ignition coil 90, as shown in waveform J, is selected to besufficient to provide a proper ignition spark. The parameters of thecharge and discharge cycles of the constant duty cycle circuit 12 andthe time multiplier circuit 16 preferably are selected to cause themultiplier ratios and the constant duty cycle ratios to be such that thecoil 90 turnon time of waveform J becomes a constant on time over thelimits of the lower speed range determined by the fixed width of theoutput pulses from the monostable multivibrator 11. Once that a speed isreached where the time multiplier circuit 16 ceases to operate, theturn-on time of the coil becomes a constant duty cycle time rather thana constant on time as described previously To maintain stable operatingvoltages in the circuit and further to provide voltage protection forthe dwell circuitry, a zener diode 100 is connected between ground andthrough the resistor 64 to the positive voltage supply terminal 42. Asecond zener diode 101 is coupled between ground and a resistor 102connected to the positive voltage supply terminal 42 and establishes thecollector potentials for the transistors 92 and 94 in the driver circuit22. In addition, another zener diode 104 is connected across thecollector and emitter of the transistor 95 to establish a maximumvoltage limit on the base of the transistor 24 that keeps the outputtransistor 24 within its safe operating area during the turn on or turnoff conditions of operation of the circuit.

Protection of the output transistor 24 from high voltage transientsproduced during the collapse of the flux in the coil 90 is provided by apair of parallelconnected reversely poled diodes 106 and 107.

The portion of the circuit which has been described thus far is all thatis necessary for normal operation of an engine to provide electronicignition for the spark plugs of the engine. It will be noted, however,that after the occurrence of each trigger pulse or output pulse from themonostable multivibrator 11, the circuit is in a condition in which thetransistor 24 is conductive and current flows through the primarywinding at the ignition coil 90. This current flow is initiated at atime determined by a prediction of the circuit as to when the nexttrigger pulse from the monostable multivibrator 11 will occur. If notrigger pulse occurs as predicted to turn off the coil circuit, theoutput transistor 24 will dissipate high power for a long period oftime. This is undesirable both as a waste of power from the batterycoupled to the terminal 42 and from the standpoint that it is harmful tothe transistor and could result in its destruction.

Therefore, it is desirable to provide a circuit to slowly turn off thecurrent through the coil if no input pulse is received from themonostable multivibrator 11 within a time interval which is greater thanthe longest interval which should occur in normal operation of thesystem. A slow tum-off is required to avoid a rapid collapse of flux inthe coil 90 which would produce a false ignition pulse. This is thefunction which is performed by the time limiter reference circuit 28.The pulses from the output of the monostable multivibrator circuit 11which are applied to the bases of the transistors 34 and 74 also areapplied through a coupling resistor to the base of an NPN transistor 111which also is connected through a resistor 112 to ground. In the absenceof any pulses from the monostable multivibrator 11, the base of thetransistor 111 is at ground potential and the transistor 111 is notconductive. Its collector potential then rises to a positive value,provided the transistor 80 is nonconductive. Whenever a pulse from themonostable multivibrator 11 appears, however, the transistor 1 1 1- isbiased into conduction and causes a near ground potential to be appliedto the base of a normally nonconductive PNP transistor 113. Thecollector potential for the transistors 1 1 1 and 80 is derived from theterminal 42 through resistors 115, 116 and 118 coupled directly to thecollector of the transistor 1 11 and through a diode 119 to thecollector of the transistor 80.

Thus, whenever either of the transistors 80 or 111 are renderedconductive, a near ground potential is applied from the collector ofthose transistors to the base of the transistor 113 to render itconductive. At all other times the transistor 113 is nonconductive.Whenever the transistor 113 conducts, it applies a charging current to acapacitor 114 (typically 20 microfarads) to rapidly charge the capacitorthrough a low impedance path. Whenever the transistors 111 and 80 bothare nonconductive, the capacitor 1 14 discharges through a highimpedance resistor 116 connected in parallel with the capacitor 114.Under normal operation of the circuit the transistor 113 is renderedconductive in each cycle of operation at least for the duration of theoutput pulses from the monostable multivibrator 11 to maintainthe chargeon the capacitor 114. Under normal operating conditions, the intervalsduring whichthe transistor 113 is nonconductive are insufficient topermit the capacitor 114 to discharge to more than a small amountthrough the resistor 116.

The junction of the capacitor 114 and the emitter of the transistor 113is connected through a coupling resistor 120 to the base of an NPNcontrol transistor 122. After the first pulse from the multivibrator 11and for normal operation, the bias applied to the base of the transistor122 from the capacitor 114 is sufficient to maintain a forward bias onthe base of the transistor 122 and it is conductive. When the transistor122 is conductive, current flows through its emitter circuit whichincludes a transistor diode 124 connected in series with a pair ofresistors 125 and 126 to ground. The junction of the transistor diode124 with the resistor 125 is coupled to the base of a further currentcontrol transistor 127 which has the sanne current flowing through it sothat the transistor 127 is conductive for normal operation of thecircuit. Whenever the transistor 127 is fully conductive, an NPN controltransistor 129, the base of which is coupled to the collector of thetransistor 127, is rendered nonconductive, and a positive potentialappears on its collector. This positive potential is applied to thecathode of the diode 86 in the AND gate 18, reverse biasing the diode66; so that the transistor 88 of the driver circuit 22 responds to theinputs applied through the other diodes 84 and 82 in the AND gate 18.

If input pulses from the monostable multivibrator 11 do not occur withina pre-established minimum time interval, the transistor 113 remainscontinuously nonconductive; and the charge on the capacitor 114 slowlyreduces as it discharges through the resistor 116. This linearly reducesthe forward bias of the transistor 122 until it becomes nonconductive.The conductivity of the transistor 127 follows that of the transistor122 to linearly increase the forward bias applied to the transistor 129to linearly increase its conduction. As the transistor 129 is renderedincreasingly conductive, the diode 86 is increasingly forward biased tolinearly reduce the conductivity of the transistor 88 to a state ofnonconduction irrespective of the inputs to the diodes 82 and 84 in theAND gate 18. Thus current flow through the primary winding of theignition coil 90 is relatively slowly reduced and terminated and thecircuit then is in a standby condition ready for the next pulse from themonostable multivibrator 11.

Current limiting is also effected by the circuit 20 when the outputpower switching transistor 24 attempts to conduct a current greater thana pre-established mount to which the current limiter circuit 20 isadjusted. To accomplish such current limiting, the emitter of theswitching transistor 24 supplies the coil current through a resistor 131to ground. The junction of the emitter of the transistor 24 with thisresistor is coupled through a resistor 130 to the emitter of thetransistor 127. The amount of current supplied by the transistor 127 isdetermined by the circuit elements connected to the transistor 122, aspreviously described.

Whenever the current flowing from the emitter of the transistor 24exceeds the current supplied by the transistor 127, the emitter of thetransistor 127 is provided with an increasing reverse bias to reduce itsconductivity. This in turn causes the potential on its collector torise. so that the transistor 129 is rendered conductive (but notsaturated) in an amount determined by the magnitude of the currentsupplied by the transistor 24 causing the reverse bias of the transistor127. As the transistor 129 commences conduction, a linear reduction inthe magnitude of the signal applied from the AND gate 18 to the base ofthe transistor 88 occurs. This causes the conductivity of the transistor88 to be limited or reduced to reduce the drive signal applied to theoutput transistor 24 thereby reducing its conductivity. This in turnresults in effecting the desired current limiting once the maximumcurrent to which the circuit is adjusted has been reached.

I claim: 1. An electronic ignition system for charging and dischargingan ignition coil to produce a spark to operate an internal combustionengine, including in combination:

Circuit means for conducting direct current through the ignition coil tocharge the same in response to a control signal;

pulsing means for producing pulses of a predetermined duration at afrequency proportional to engine RPM;

coincidence gate means with at least first and second inputs, and anoutput coupled with said circuit means and producing said control signalon the output thereof upon coincidence of a predetermined relationshipof signals on said first and second inputs thereof;

constant duty cycle means having an input coupled with said pulsingmeans and having an output, said duty cycle means responsive to saidpulses for producing an output signal on the output thereof indicativeof a first state for a predetermined precent of the time intervalbetween the beginning of successive pulses from said pulsing means andindicative of a second state the remainder of said time interval betweensaid pulses, the output of said duty cycle means coupled with the firstinput of said gate means;

time multiplying means operated in response to said pulsing means andsaid constant duty cycle means, having an input coupled with saidpulsing means and further coupled with the output of said duty cyclecircuit means, and having an output, said time multiplying circuit meansproducing a gate inhibiting signal on said output, said gate inhibitingsignal commencing with the output of said constant duty cycle meanschanging from an output indicative of said first state to an outputindicative of said second state and said inhibiting signal having avariable time period which has a predetermined relationship with thetime interval extending from the end of a pulse from said pulsing meansuntil the output signal of said duty cycle circuit means changes fromsaid first state to said second state; and

means for coupling the output of said time multiplying means with thesecond input of said gating means, said gating means producing saidcontrol signal upon coincidence of an output indicative of said secondstate of operation from said constant duty cycle means and terminationof said gate inhibiting signal from said time multiplying means.

2. The combination according to claim 1 wherein said time multiplyingmeans includes means for causing said variable time period of said gateinhibiting signal to be a predetermined multiple of the time periodextending from the end of a pulse from said pulsing means until theoutput of said duty cycle circuit means changes from an outputindicative of said first state to one indicative of said second state,no gate inhibiting signal being produced whenever said pulse from saidpulsing means has a duration greater than the duration of an outputsignal indicative of said first state on the output of said constantduty cycle means.

3. The combination according to claim 1 wherein said pulsing meansincludes a monostable multivibrator, the output of which is coupled withthe inputs of said constant duty cycle means and said time multiplyingmeans.

4. The combination according to claim 1 wherein said coincidence gatemeans has a third input and further including a time interval measuringmeans coupled with the output of said pulsing means and having an outputcoupled with the third input of said coincidence gate means, said timeinterval measuring means normally enabling said coincidence gate meansand producing an inhibiting signal on the output thereof in response toa predetermined time interval between successive pulses from saidpulsing means.

14 the output of said pulsing means and having an output coupled withsaid current limiter means for causing said current limiter means toapply an inhibiting signal to the third input of said gate means toterminate said control signal in response to a predetermined timeinterval between successive pulses from said pulsing means.

1. An electronic ignition system for charging and discharging anignition coil to produce a spark to operate an internal combustionengine, including in combination: Circuit means for conducting directcurrent through the ignition coil to charge the same in response to acontrol signal; pulsing means for producing pulses of a predeterminedduration at a frequency proportional to engine RPM; coincidence gatemeans with at least first and second inputs, and an output coupled withsaid circuit means and producing said control signal on the outputthereof upon coincidence of a predetermined relationship of signals onsaid first and second inputs thereof; constant duty cycle means havingan input coupled with said pulsing means and having an output, said dutycycle means responsive to said pulses for producing an output signal onthe output thereof indicative of a first state for a predeterminedprecent of the time interval between the beginning of successive pulsesfrom said pulsing means and indicative of a second state the remainderof said time interval between said pulses, the output of said duty cyclemeans coupled with the first input of said gate means; time multiplyingmeans operated in response to said pulsing means and said constant dutycycle means, having an input coupled with said pulsing means and furthercoupled with the output of said duty cycle circuit means, and having anoutput, said time multiplying circuit means produciNg a gate inhibitingsignal on said output, said gate inhibiting signal commencing with theoutput of said constant duty cycle means changing from an outputindicative of said first state to an output indicative of said secondstate and said inhibiting signal having a variable time period which hasa predetermined relationship with the time interval extending from theend of a pulse from said pulsing means until the output signal of saidduty cycle circuit means changes from said first state to said secondstate; and means for coupling the output of said time multiplying meanswith the second input of said gating means, said gating means producingsaid control signal upon coincidence of an output indicative of saidsecond state of operation from said constant duty cycle means andtermination of said gate inhibiting signal from said time multiplyingmeans.
 2. The combination according to claim 1 wherein said timemultiplying means includes means for causing said variable time periodof said gate inhibiting signal to be a predetermined multiple of thetime period extending from the end of a pulse from said pulsing meansuntil the output of said duty cycle circuit means changes from an outputindicative of said first state to one indicative of said second state,no gate inhibiting signal being produced whenever said pulse from saidpulsing means has a duration greater than the duration of an outputsignal indicative of said first state on the output of said constantduty cycle means.
 3. The combination according to claim 1 wherein saidpulsing means includes a monostable multivibrator, the output of whichis coupled with the inputs of said constant duty cycle means and saidtime multiplying means.
 4. The combination according to claim 1 whereinsaid coincidence gate means has a third input and further including atime interval measuring means coupled with the output of said pulsingmeans and having an output coupled with the third input of saidcoincidence gate means, said time interval measuring means normallyenabling said coincidence gate means and producing an inhibiting signalon the output thereof in response to a predetermined time intervalbetween successive pulses from said pulsing means.
 5. The combinationaccording to claim 1 further including current limiter means coupledwith said circuit means and a third input of said gate means andresponsive to current in excess of a predetermined value in said circuitmeans for applying a signal to the third input of said gate means toeffect a change in said control signal.
 6. The combination according toclaim 5 further including a time interval measuring means coupled withthe output of said pulsing means and having an output coupled with saidcurrent limiter means for causing said current limiter means to apply aninhibiting signal to the third input of said gate means to terminatesaid control signal in response to a predetermined time interval betweensuccessive pulses from said pulsing means.